BEIRUT — More than 2,000 Syrians — almost half of them pro-government forces — have been killed in just over two weeks of fighting in Syria, marking one of the worst death tolls in the country's three-year civil war, opposition activists said Monday.

The reports reflect a recent surge in deadly attacks by the al-Qaida-breakaway Islamic State group targeting President Bashar Assad's forces, signaling shifting priorities as Sunni militants seek to consolidate their hold on territory and resources in northern Syria.

Assad's forces have gained momentum in the fighting with rebels seeking to topple him from power. Infighting also has hurt the rebel cause, with Islamic extremists battling more moderate fighters who have been greatly weakened by lack of weapons and clashes with the militants.

But a series of recent setbacks for the Syrian government at the hands of the Islamic State group threatens to overturn government successes, pitting the Syrian army against a formidable force that now controls large chunks of territory in the country's north and neighboring Iraq.

"Now that they've mopped up rebel resistance to them in the east, the Islamic State (group) can turn to the regime," said Aymenn al-Tamimi, an expert on militant factions in Syria and Iraq. "It may have been a benefit (to the Islamic State) to deal with rebels first, but the assault against the regime was inevitable."

The recent attacks came after Assad was re-elected last month to a third, seven-year term in a vote that was confined to government-controlled areas and dismissed by the opposition and its Western allies. In his inauguration speech on July 16, he confidently declared victory and praised his supporters for "defeating the dirty war."

Since then, fighters from the Islamic State group have launched attacks against army positions in three different provinces in northern and central Syria. In the past week alone, the militants captured a government-controlled gas field and two major army bases in three different provinces.

More than 300 soldiers, guards and workers at the Shaer field were reported killed by Islamic State militants in a three-day offensive to capture the field. The army recaptured Shaer this past weekend.

Militants last week also overran the sprawling Division 17 military base in the northern Raqqa province, killing at least 85 soldiers inside. Amateur videos posted online by activists showed more than a dozen beheaded bodies in a busy square said to be in Raqqa. Some of the heads were placed on a nearby fence, where at least two headless bodies were crucified. The videos appeared genuine and corresponded to other AP reporting of the events.

On Sunday, the militants seized the army's Regiment 121 at Maylabieh in the northern Hassakeh province after a three-day battle.

Beyond Syria, the Islamic State fighters have seized large swaths of land in northern and western Iraq and have declared a self-styled caliphate across territory straddling the Iraq-Syria border.

In the past, Islamic State fighters and government forces have largely avoided engaging each other, triggering accusations among mainstream Syrian rebels fighting to topple Assad that the two sides were colluding against them.

Those accusations have been blunted by the recent fighting, which suggests the Islamic State is fighting on all fronts in its quest to expand its territory.

The group is also engaged in heavy fighting against rival, mainstream Syrian rebels, and against Kurdish fighters in northern Syria. In their push, the Islamic State fighters have captured much of Syria's oil-rich eastern province of Deir el-Zour, which borders Iraq, and are now fighting for control of parts of Hassakeh province in the north.

Analysts said the militant attacks against government forces, racking up such high casualties, were in part to prove they're still committed to the fight against Assad.

"It seems to be they are trying to consolidate their territory and tighten up their weak spots," said Christopher Davidson, a professor of Middle East Politics at Durham's School of Government and International Affairs.

The Britain-based Syrian Observatory for Human Rights said more than 2,000 people have been killed since Assad's inauguration, nearly half of them soldiers and government-allied militiamen.

It didn't provide a breakdown for the rest of the casualties, which would include civilians and opposition fighters.